Vacuum cleaners, in particular electric vacuum cleaners designed for use in the home, nearly exclusively use multi-stage particulate filters. Typically, a pre-filter, preferably in the form of a dust bag, is provided upstream of a vacuum cleaner fan, and an exhaust filter is disposed downstream thereof to remove fine dust that has passed through the dust bag. The exhaust filter also collects, for example, the particles which are abraded from the carbon brushes or the like of the drive motor of the fan. Both filter stages are consumable items, which need to be replaced by a user of the vacuum cleaner. To this end, it is helpful to give the user a reliable and usage-based indication of when the bag or the exhaust filter needs to be replaced as a result of an upper limit for the filling level or saturation being reached. For the dust bag, it is common to use sensors which operate based on the differential pressure principle. For the exhaust filter, the time of use of the filter can be used as a criterion for determining when replacement is necessary. For this purpose, a time counter may be used which is manually reset by the user after insertion of the exhaust filter and which, after a predetermined operating time has elapsed, indicates that the filter needs to be replaced. The algorithm and the controls and indicators needed for this are implemented in and form part of a vacuum cleaner control system.
German Patent Publication DE 102 29 796 describes a filter having a usage indicator which operates based on temperature-dependent integration. The color of the indicator changes in a temperature-dependent manner each time the vacuum cleaner is used for a prolonged period of time. German Patent Publication DE 602 05 753 T2 describes time-dependent usage indicators which need to be activated by a user by opening a liquid reservoir. A colored indicator liquid diffuses into an absorbent material which is provided in the usage indicator and which then changes color as a function of time and, therefore, is a measure for the period of use.
FIG. 1 shows a conventional vacuum cleaner 10 having a dust chamber 12 in which may be positioned a dust bag 14. Dust 18 is conveyed through a suction hose 16 to dust bag 14 and collected therein. Dust 18 is transported by air flow 20, which is generated by a fan 22 (vacuum cleaner fan). Dust chamber 12 is closed by a dust chamber cover 24. Finer fractions of dust 18, which pass through dust bag 14, are carried into a vacuum cleaner exhaust filter 26 by the exhaust air or vacuum air flow 20 of fan 22.
Vacuum cleaner 10 includes a control processor 28, which is in operative connection with a control and display unit 30 disposed on vacuum cleaner 10. The signals from control and display unit 30 are used by control processor 28 to adjust the suction power of fan 22, and thus, the amount of dust 18 that can be picked up by vacuum cleaner 10.
In some designs of vacuum cleaners 10, an indication of an upcoming need to replace vacuum cleaner exhaust filter 26 is provided by control and display unit 30 based on, for example, the accumulated operating time of vacuum cleaner 10, which is determined by control processor 28. In a vacuum cleaner 10 having such a function, the operating time meter is reset via control and display unit 30 after replacement of vacuum cleaner exhaust filter 26.
However, when the time of use is used as a criterion for determining when a filter needs to be replaced, the load actually placed on the filter is not, or not optimally, taken into account, because the linear time progression alone is not able to reproduce the actual usage behavior, which varies over time.